Cell cracking factor analysis

Analysis of cracking factors of photovoltaic cells before lamination of photovoltaic modules
Before the lamination of photovoltaic modules, the defects in the semi-finished components are checked in time and reworked by EL test, which is a key link in the production of photovoltaic modules. Common rework defects are cracking, soldering, and others. Among them, the reciprocal proportion of the battery sheet is the highest. Factors causing cracks in the battery include: cell crystal type, ambient temperature and humidity, soldering station temperature, cleanliness, laying methods, etc. All-round attention and corresponding measures are effective ways to reduce the crack of the battery.
In the production process of photovoltaic modules, rework is performed due to some abnormality or defect in the finished component (or semi-finished component). The quality control points for finding abnormalities or defects are mainly divided into three steps: EL test before lamination, visual inspection after lamination, and final EL measurement. After lamination, the glass, EVA, battery sheet, EVA, and back sheet are combined into a whole under the action of high temperature and high pressure. It is difficult to disassemble under normal conditions, and rework is difficult. Therefore, it is especially important to check, find and change the defects in the semi-finished components in time by EL test before lamination.
The process before lamination includes the welding and laying of the battery sheets, and each process may have a bad phenomenon due to factors such as equipment, people, materials, methods, and environment. Common unfavorable phenomena caused by the welding process include: cell cracking, soldering, strip deviation, tin wire, tin slag, tin plating, flux crystallization residue, etc. Common unfavorable phenomena caused by laying process include: cell crack, foreign matter, parallel film, small string spacing, poor backplane, backing strips are not centered, bus bar spacing is small, bus bar bending, bus bar soldering, The length of the bus bar is different, the bar code is poorly attached, and the counter is reversed. According to the frequency of defects in the EL test before lamination, the rework caused by the above defects is roughly classified into three categories:
The first category: crack rework. The frequency is the highest, and the welding and laying processes may cause the battery to crack.
The second category: virtual welding rework. The virtual welding is a bad process in the welding process, and it is necessary to find and adjust the automatic welding machine equipment in time.
The third category: artificial rework. The general term for other unhealthy phenomena, collectively referred to as artificial rework, because each type of failure occurs less frequently and is mostly caused by carelessness.
Figure 1 is a line drawing of the rework ratios of different types of EL test before lamination of single crystal components produced by the author's company in May 2016. It can be seen that the proportion of rework caused by cracking is the highest, the proportion of rework caused by man-made is second, and the proportion of rework caused by virtual welding is the lowest.

This paper analyzes and studies the cracking factors caused by rework of semi-finished components before lamination, and proposes rectification measures.
The reasons for the cracking of the cell sheet before lamination are various, and the following is analyzed item by item.
1 single poly type

In the single crystal cell, all atoms of silicon are arranged in short-range order and long-range and ordered; in polycrystalline cells, the arrangement of silicon atoms is only short-range and ordered, and there are grain boundaries (as shown in Fig. 2) [1]. Due to the certain force between the polycrystalline grain boundaries, when subjected to an external force, the polycrystalline cell sheet will not be broken to some extent under the interaction force between the grain boundaries. In contrast, monocrystalline cells are more brittle than polycrystalline cells and have a high degree of cracking.

For automatic welding, since the force of the battery sheet is relatively uniform, the ratio of cracking of the single polycrystalline battery sheet is not much different; but for manual welding, the ratio of cracking of the single crystal is high due to the difference in artificial strength. This is caused by the difference in the internal structure of the single and polycrystalline cells.
2 ambient temperature and humidity

The instructions for the welding machine used by the company in which the company is used indicate that if the ambient temperature is too low, the fragmentation rate will increase; when the humidity is increased, the insulation will be cracked, the control components will be damaged, and the humidity will cause condensation. On the other hand, when the humidity is increased, the warpage of the battery sheet is increased, thereby increasing the proportion of cracking. At a maximum temperature of +40 ° C, the relative humidity of the air does not exceed 50%; at lower temperatures, higher humidity is allowed, for example, 90% at 20 ° C. It is recommended that the ambient air temperature be: 10-40 oC, 24 h, the average value does not exceed +35 ° C; the humidity is 30% to 75%. Special measures should be taken for the occasional condensation due to temperature changes. There is condensation.
3 welding machine

Compared with manual welding, automatic welding reduces the number of operations on the battery, and achieves simultaneous welding of single and series welding [2]. However, when the parameters of the welding machine and the various components are improperly adjusted, the battery sheet may be cracked. It mainly includes the following aspects:
3.1 fixed station temperature

The fixed platform is divided into five areas: a preheating station, a welding station, a first cooling zone, a second cooling zone, and a third cooling zone. Before the welding of the battery piece, it will enter the welding station through the preheating zone and be welded by high frequency induction welding head. The heating rate, holding time and cooling speed of the welding are strictly controlled by gradient, and the temperature is preheated by three gradients before welding. After welding, the temperature is gradually lowered by four gradients. It is recommended that the temperature of the preheating station is 50~60°C, the temperature of the soldering station is 110～140°C, the temperature of the cooling zone is 90-100°C, the temperature of the cooling zone 2 is lower than that of the cooling zone, and the temperature of the cooling zone 3 is lower than the cooling zone. temperature. The battery sheet is brittle and brittle, and rapidly heats up or cools down, causing the battery sheet to be heated and cooled, causing thermal expansion and contraction. In addition, the welding exerts a certain force, and the battery sheet is easily broken. Therefore, strengthening the temperature detection of the preheating station, the welding station, and the cooling zone is to reduce the crack of the cell.
One of the ways.
3.2 Cleanliness

Moving table tops, welding table tops, vacuum chucks, conveyor belts, etc., but wherever the battery contacts, as long as there is foreign matter, it may adhere to the surface of the battery, causing cracks. In addition to the battery chip slag, the main foreign matter is the flux crystallization residue. Therefore, it is particularly important to regularly clean the crystallization of the welding head (including the front welding head, the middle welding head, and the rear welding head) and keep the surface of the welding machine clean.
In addition, Teflon tape is adhered to the soldering station. When the bonded Teflon tape has a wing on the side after welding for a period of time, it must be replaced. When pasting, no air must remain under the blanket.
3.3 uneven base surface and uneven suction force of the nozzle

The unevenness of the welding platform or the unevenness of the nozzles that adsorb the battery sheets can also cause the battery sheets to crack. These two undesirable factors, such as happening on the typesetting machine, can also cause the battery to crack.
4 laying methods
After the battery piece is soldered and string welded, it must go through the laying process. The above materials are stacked in the order of glass, EVA, battery string, EVA, and back sheet. In order to avoid battery skew during stacking, the battery string is fixed with high temperature tape. On the other hand, in order to collect current, the bus bar and the battery string lead wire are soldered with a soldering iron tip. In this process, due to strength, technique, etc., it may cause certain damage to the battery. The author analyzed the position of 708 lobes in 9211 single crystal modules (6x12) produced by the company in June 2016, as shown in Figure 3.

Fig. 3 Distribution of cracked position of EL test cell before lamination
It can be seen that the positions of the lobes are mainly concentrated in D1, C1, E12, A1, D11, E3, B10, A12, D5, B1, a total of 10 positions. Through detailed analysis of the location with high probability of cracking and prominent causes, the causes of related cracks and targeted countermeasures are found, as shown in Table 1.

Through analysis, it is found that the location with high probability of cracking is the place where the frequency of human contact is high. Therefore, it is the future direction to improve the training of employees' standardization work and strengthen the techniques.
5 raw materials

Raw materials that cause the cell to crack, including the ribbon and the cell itself.
5.1 ribbon distortion

The machine is soldered with a shaft-mounted soldering strip. When there is a single distortion on the solder ribbon, it will definitely cause the battery to crack. Therefore, it is necessary to strengthen the product requirements of the welding tape supplier and strengthen the inspection. It is necessary to make a record of loading.
5.2 The battery chip itself has a large crack ratio

In actual production, it was found that there was a case where the crack ratio of a certain batch of a certain cell was too high. Therefore, the EL blank crack test is performed on the incoming material of the battery in real time, the management of the incoming material of the battery sheet is strengthened, the loading record is made, the supplier is communicated in time, the batch replacement or the separate box replacement is performed, and the lamination is also reduced. An effective way to retire rework.
Reducing the ratio of crack rework of semi-finished components before lamination requires a lot of aspects. First of all, strictly check the quality of incoming materials such as battery sheets and solder ribbons, and minimize the hidden troubles of the incoming materials to the semi-finished components. Secondly, strengthen the control of the ambient temperature and humidity, the control of the temperature of the welding machine fixed table, the treatment of the cleanliness, and the management of the flatness of the welding machine and the strength of the nozzle. Once again, it is also essential to pay attention to the methods of laying the operators and strengthen the training. The position is accurate and the intensity is soft. It is also necessary to cultivate good operating habits and quality awareness.